The existing discussion of HTTP Response Codes in Section 5.5 is pretty thin in that it does not provide any useful information about what information a client should expect back in the event of an error condition. This maintains the current status quo of only expecting a HTTP status code but "strongly recommends" that a natural language description of the error be included in the response.
relative to -06 draft (replace the existing section 5.5 with)
5.5 HTTP Response codes
APP uses the HTTP defined response status codes to indicate the success or failure of an operation. Consult the HTTP specification [RFC2616] for detailed definitions of each status code. It is strongly recommended that entities contained within HTTP 4xx and 5xx responses include a human readable, natural language explanation of the error.